Scanning image display apparatus

ABSTRACT

A scanning type image display apparatus is disclosed which is capable of forming an exit pupil with a shape and a size which facilitate observation. The apparatus includes a scanning unit, a first optical system introducing a light flux to the scanning unit, a second optical system converging the light flux from the scanning unit, a diffractive optical element receiving the converged light flux, and an ocular optical system. Two directions in diametral directions of the light flux are first and second directions. The optical element increases the divergent angles of an emerging light flux therefrom in the first and second directions as compared with the convergent angles of an incident light flux thereinto in the two directions and increases the divergent angle of the emerging light flux in the first direction as compared with the divergent angle thereof in the second direction.

BACKGROUND OF THE INVENTION

The present invention relates to a scanning type image display apparatuswhich, mainly, is mounted on the head of a user and allows the user toobserve an image by scanning a light flux on an eye (retina) of theuser.

A retina|scanning type image display apparatus has been disclosed, forexample, in U.S. Pat. No. 5,467,104, Japanese Patent Laid-Open No.2004-347687, and U.S. Pat. No. 6,157,352. In those retinal scanning typeimage display apparatuses, a scanning device and an ocular opticalsystem are used to perform two-dimensional scanning of red light, greenlight, and blue light from a light source on a retina of an observer toallow observation of an image.

In those retinal scanning type image display apparatuses, a scanningdevice having a small size and capable of fast scanning of light is usedin order to realize a higher resolution. Since such a small scanningapparatus is used, a light beam for scanning needs to have a very smalldiameter.

This involves the problem of the likelihood of vignetting in imagessince the diameter of the light beam is also small in the exit pupilarea in which the eye of an observer is placed. A method for increasingthe diameter of the exit pupil is to dispose an exit pupil expander inan optical system of a retinal scanning type image display apparatus asdisclosed in U.S. Pat. No. 6,157,352. Specifically, an exit pupilexpander such as a lens array and a diffusing plate is disposed close tothe position of the intermediate image plane of the optical system toprovide the divergent angle of a light flux emerging from the exit pupilexpander that is larger than the divergent angle of the light fluxentering the exit pupil expander, thereby increasing the diameter of theexit pupil.

When the exit pupil expander as disclosed in U.S. Pat. No. 6,157,352 isused, however, the following problem arises if the emergence angle ofthe light emerging from the exit pupil expander is extremely larger thanthe angle of the light entering the exit pupil expander. Specifically,although the diameter of an area (exit pupil) for allowing an observerto see an image is increased, the amount of light reaching the eyes ofthe observer is relatively small to present a darker image. In addition,the diffused light may serve as stray light to make it difficult toperform favorable image observation.

The diameter (width) of the exit pupil formed by the light emerging fromthe exit pupil expander in a certain direction is equal to that in adirection orthogonal thereto. Thus, when a rectangular image isdisplayed, a loss of light amount is not satisfactorily small in thatoptical system.

BRIEF SUMMARY OF THE INVENTION

The present invention provides a scanning type image display apparatuscapable of forming an exit pupil with a shape and a size whichfacilitate observation by an observer and achieving a reduced loss oflight from a light source.

A scanning type image display apparatus as one aspect of the presentinvention includes a scanning unit which two-dimensionally scans a lightflux, a first optical system which introduces a light flux from a lightsource to the scanning unit, a second optical system which converges thelight flux from the scanning unit, a diffractive optical element whichreceives the converged light flux from the second optical system, and anocular optical system which introduces the light flux from thediffractive optical element to an eye of an observer. When twodirections orthogonal to each other in diametral directions of the lightflux from the second optical system are a first direction and a seconddirection, the diffractive optical element has a function of increasingthe divergent angles of an emerging light flux from the diffractiveoptical element in the first and second directions as compared with theconvergent angles of an incident light flux entering the diffractiveoptical element in the first and second directions and a function ofincreasing the divergent angle of the emerging light flux in the firstdirection as compared with the divergent angle thereof in the seconddirection.

An image display system as another aspect of the present inventionincludes the above-described image display apparatus and an image supplyapparatus.

Other objects and features of the present invention will become apparentfrom the following description and the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a figure showing the configuration of a retinal scanning typeimage display apparatus which is Embodiment 1 of the present invention.

FIG. 2 is a figure for explaining how to form an image in the retinalscanning type image display apparatus of Embodiment 1.

FIG. 3A is a figure for explaining the optical effect in a horizontalsection of a diffractive optical element used in the retinal scanningtype image display apparatus of Embodiment 1.

FIG. 3B is a figure for explaining the optical effect in a verticalsection of the diffractive optical element in Embodiment 1.

FIG. 4 is a figure for explaining the optical effect of the retinalscanning type image display apparatus of Embodiment 1.

FIG. 5A is a figure showing the shape of the diffractive optical elementin Embodiment 1.

FIG. 5B is a figure showing the sectional shape of the diffractiveoptical element in Embodiment 1.

FIG. 5C is a figure showing the sectional shape of the diffractiveoptical element in Embodiment 1.

FIG. 6 is a figure showing a diffraction pattern provided by thediffractive optical element in Embodiment 1.

FIG. 7 shows the relationship between a periodic structure and a spot ofincident light in the diffractive optical element in Embodiment 1.

FIG. 8A is a figure showing the relationship between a light flux and aneye in observation of the center of an image.

FIG. 8B is a figure showing the relationship between a light flux andthe eye in observation of the end of the image.

FIG. 9 is a figure showing a head-mounted image display apparatus inwhich the retinal scanning type image display apparatus of Embodiment 1is used.

FIG. 10 is a figure showing the shape of a diffractive optical elementused in a retinal scanning type image display apparatus which isEmbodiment 2 of the present invention.

FIG. 11 is a figure showing a diffraction pattern provided by thediffractive optical element in Embodiment 2.

FIG. 12 is a figure showing the configuration of a retinal scanning typeimage display apparatus which is Embodiment 3 of the present invention.

FIG. 13A is a figure showing the shape of a diffractive optical elementused in the retinal scanning type image display apparatus of Embodiment3.

FIG. 13B is a figure showing the sectional shape of the diffractiveoptical element in Embodiment 3.

FIG. 13C is a figure showing the sectional shape of the diffractiveoptical element in Embodiment 3.

FIG. 14 is a figure showing a diffraction pattern provided by thediffractive optical element in Embodiment 3.

DESCRIPTION OF THE EMBODIMENTS

Embodiments of the present invention will hereinafter be described withreference to the accompanying drawings.

Embodiment 1

FIG. 1 shows the configuration of a retinal scanning type image displayapparatus which is Embodiment 1 of the present invention. A light source1 is capable of modulating the intensity of light which is emittedtherefrom. A light flux emitted from the light source 1 passes through afirst optical system 2 which serves as a light source optical system,and then enters a two-dimensional scanning unit 3.

The two-dimensional scanning unit 3 is a light deflector of a reflectiontype formed of a Micro Electro-Mechanical System (MEMS) manufactured byusing semiconductor-manufacturing processes.

The light flux deflected by the two-dimensional scanning unit 3 enters asecond optical system 4 which serves as a collecting optical system. Thesecond optical system 4 converges the light flux from the light source 1at the position where a diffractive optical element 5 is placed, andforms a scanned surface. Light fluxes 8 a, 8 b, and 8 c are examples ofthe light flux deflected by the two-dimensional scanning unit 3.

Preferably, the position of the diffractive optical element 5 coincideswith the position of the scanned surface. However, they do notnecessarily need to coincide with each other. They only need to beplaced within an area in which they can be considered to opticallycoincide with each other.

The light flux after the diffraction by the diffractive optical element5 enters an ocular optical system 6 via a reflecting mirror 9. Anobserver puts his eye in the area where an exit pupil 7 is formed by thelight flux emerging from the ocular optical system 6, so that he/she canobserve an image formed on the abovementioned scanned surface as avirtual image through the ocular optical system 6. ‘AXL’ represents theoptical axis of the optical systems of the image display apparatus. Theoptical axis AXL corresponds to the optical path on which a principalray proceeds from the light-emitting point of the light source 1 to thecenter of the exit pupil 7 after passing through the centers of thefirst and second optical systems 2 and 4.

The optical source 1 is electrically connected to a light source controlcircuit 10. The two-dimensional scanning unit 3 is electricallyconnected to a scanning control circuit 11. An image information inputcircuit 13 is electrically connected to an image supply apparatus 50such as a personal computer, a DVD player, and a television tuner. Theimage display apparatus and the image supply apparatus 50 constitute animage display system.

The image information input circuit 13 outputs a signal corresponding toimage information input thereto from the image supply apparatus 50 to amain control circuit 12. The main control circuit 12 controls the lightsource control circuit 10 and the scanning control circuit 11 based onthe signal from the image information input circuit 13 such that thecircuits 10 and 11 are synchronized. In this manner, the imagecorresponding to the image information input from the image supplyapparatus 50 can be presented to the observer.

Description will hereinafter be made of how to make an image by thelight source 1 and the two-dimensional scanning unit 3 in the retinalscanning type image display apparatus of Embodiment 1 with reference toFIG. 2.

The light flux emitted from the light source 1 enters thetwo-dimensional scanning unit 3 via the first optical system 2.Specifically, the light flux enters a deflecting mirror surface 3 c inthe two-dimensional scanning unit 3.

The deflecting mirror surface 3 c is held by a substrate 3 w via torsionbars 3 a and 3 b which extend in directions orthogonal to each other.Upon application of electromagnetic force or electrostatic force from anactuator, not shown, the deflecting mirror surface 3 c is rotated aroundthe two orthogonal axes with twisting of the torsion bars 3 a and 3 b tochange the reflection direction of the incident light flux. The torsionbars 3 a and 3 b, and the deflecting mirror surface 3 c are formedintegrally with the substrate 3 w by using the MEMS technology describedabove. Thus, the two-dimensional scanning unit 3 is extremely small andcapable of fast operation.

The light flux deflected (reflected) by the two-dimensional scanningunit 3 configured as above enters the second optical system 4.

The light-emitting point of the light source 1 is conjugate to a scannedsurface 5 s via the first optical system 2 and the second optical system4. The image of the light source 1 is formed as a spot 8 a in FIG. 2,for example. The two-dimensional scanning unit 3 can deflect the lightflux in two directions 3 x and 3 y orthogonal to each other, that is,two-dimensionally scan the light flux. For example, as shown in FIG. 2,the two-dimensional scanning unit 3 can perform raster scanning with ascanning line 47 and a flyback line 48 on the scanned surface 5 s. Thetwo-dimensional scanning can be synchronized with the modulation of thelight source 1 to form an image on the scanned surface 5 s.

Next, the effect of the diffractive optical element 5 will be describedwith reference to FIGS. 3A and 3B which schematically show a light fluxaround the diffractive optical element 5 in a section in a horizontaldirection (H) serving as a first direction and in a section in avertical direction (V) serving as a second direction, respectively.

The horizontal and vertical directions are also two directionsorthogonal to each other in the diametral directions of the light flux.

The horizontal direction corresponds to a direction of the long side ofa rectangular image presented to the observer, while the verticaldirection corresponds to a direction of the short side of the image.However, the horizontal direction may correspond to the short sidedirection of the image and the vertical direction may correspond to thelong side direction of the image.

In FIGS. 3A and 3B, a light flux 8 d shows an example of a light fluxentering the diffractive optical element 5. The incident light flux 8 dproceeds toward the diffractive optical element 5 while it converges atan angle of θ_(IN) (hereinafter referred to as a convergent angle inEmbodiment 1) in the horizontal direction and the vertical directionwith respect to the central axis (shown by a dash dotted line in FIGS.3A and 3B and coincident with the optical axis AXL in FIGS. 3A and 3B),and forms a convergence point at the position where the diffractiveoptical element 5 is placed. The convergent angles of the incident lightflux 8 d are the same for the horizontal direction and the verticaldirection.

The diffractive optical element 5 has a minute periodic structure 5 aformed thereon. The periodic structure 5 a diffracts the incident lightflux 8 d. In the horizontal direction shown in FIG. 8A, light rays 8 e,8 f, and 8 g represent part of the diffracted light flux (emerging lightflux) emerging from the diffractive optical element 5. The emerginglight flux represented by the light rays 8 e, 8 f, and 8 g diverges toform an angle θ_(HOUT) (hereinafter referred to as a divergent angle)larger than the convergent angle θ_(IN) of the incident light flux 8 dwith respect to the central axis.

In the vertical direction shown in FIG. 3B, light rays 8 h, 8 i, and 8 jrepresent part of the diffracted light flux (emerging light flux)emerging from the diffractive optical element 5. The emerging light fluxrepresented by the light rays 8 h, 8 i, and 8 j diverges to form adivergent angle θ_(VOUT) larger than the convergent angle θ_(IN) of theincident light flux 8 d with respect to the central axis.

The following relationships hold:

θ_(IN)<θ_(HOUT)

θ_(IN)<θ_(VOUT)

θ_(VOUT)<θ_(HOUT.)

That is, the diffractive optical element 5 has the function ofincreasing the divergent angles θ_(VOUT) and θ_(HOUT) of the emerginglight flux in the vertical and horizontal directions as compared withthe convergent angle(s) θ_(IN) of the light flux entering thediffractive optical element 5 in the vertical and horizontal directions.The diffractive optical element 5 also has the function of increasingthe divergent angle θ_(HOUT) of the emerging light flux in thehorizontal direction as compared with the divergent angle θ_(VOUT) inthe vertical direction.

FIG. 4 shows the optical systems shown in FIG. 1 developed at theposition of the reflecting mirror 9. The emerging light flux having thedivergent angle larger than the convergent angle through the diffractiveoptical element 5 as described above forms the exit pupil 7 via theocular optical system 6. The size PH of the exit pupil 7 in thehorizontal direction (H) is shown in an upper portion of FIG. 4. Thesize PV of the exit pupil 7 in the vertical direction (V) is shown in alower portion of FIG. 4. PH and PV have the following relationship:

PV<PH.

FIG. 5A shows an example of the shape of the diffractive optical element5. The diffractive optical element 5 is formed of a transmissivemicro-lens array including a plurality of micro-lenses (hereinafterreferred to as micro-lenses 5 a) arranged regularly as the periodicstructure 5 a shown in FIGS. 3A and 3B. The micro-lenses 5 a arearranged with a first periodic pitch 14 a in a horizontal direction 18 aand with a second periodic pitch 14 b larger than the first periodicpitch 14 a in an inclined direction 18 b with respect to the horizontaldirection 18 a and a vertical direction. The micro-lenses 5 a arearranged with a third periodic pitch (a combined periodic pitch of thefirst periodic pitch 14 a and the second periodic pitch 14 b) 14 clarger than the second periodic pitch 14 b in the vertical direction.

FIGS. 5B and 5C show the sectional shapes in the directions 18 a and 18b for the periodic pitches 14 a and 14 b, respectively. The micro-lensarray is formed of an optically transparent optical material andproduces the light-diffractive effect with phase distribution providedfor the light transmitted through each of the micro-lenses.

FIG. 6 shows a pattern 16 of the diffracted light emerging from thediffractive optical element 5 shown in FIG. 5A. In FIG. 6, a direction16 x corresponds to the horizontal direction (first periodic pitchdirection), while a direction 16 y orthogonal to the direction 16 xcorresponds to the vertical direction (third periodic pitch direction).As seen from FIG. 6, the diffracted light expands differently for thedirection 16 x and the direction 16 y such that the expansion of thediffracted light in the direction 16 x is larger than that of thediffracted light in the direction 16 y.

The different periodic pitches in the different directions in theperiodic structure on the diffractive optical element 5 can providedifferent distributions of light amount in the direction 16 x and thedirection 16 y such that the distribution in the direction 16 x islarger than the distribution in the direction 16 y. As a result, the useefficiency of light can be increased.

FIG. 7 shows the relationship between the periodic structure(micro-lenses) 5 a on the diffractive optical element 5 and the beamdiameter D of the incident light flux 8 d converged by the secondoptical system 4. Since the diffractive effect of light is provided bythe periodic structure, the beam diameter (spot size) D in a certaindirection on the periodic structure is desirably larger than the pitch pof the periodic structure in that direction. If the pitch p of theperiodic structure is excessively smaller than the beam diameter D, thediffraction angle per order of diffraction is excessively increased, sothat a loss of light amount tends to occur. Thus, the following isdesirably satisfied:

1<D/p<5.

The beam diameter (spot size) D is defined by Full Width Half Maximum(FWHM) or 1/e² of the peak light amount.

Next, description will be made of how to specify the shape of the exitpupil. FIGS. 8A and 8B show the relationship between an eyeball 34 of anobserver placed on the exit pupil and light fluxes 35 and 36 travelingtoward the eyeball 34 on a horizontal section. FIG. 8A shows theobserver looking at the center of an image, while FIG. 8B shows theeyeball 34 rotated to look at the end of the image.

To prevent blocking of the light flux 35 forming the center of the imageeven in the state of FIG. 8B, the light flux diameter (light flux width)φH is necessary in the horizontal direction as shown in FIG. 8A. On theother hand, in the vertical direction, the light flux diameter φV isnecessary to avoid blocking of the light flux 35. The pupil diameter ofthe observer is typically set to 3 to 5 mm.

Typically, the image viewed by the observer has a dimension in a ratioof 4:3 or 16:9 between the lateral or horizontal direction and thelongitudinal or vertical direction, and thus the image is horizontallyoriented. The following needs to be satisfied:

1<φH/φV   (1)

FIG. 9 shows a head-mounted display (head-mounted image displayapparatus) 43 which is configured by placing a pair of scanning typeimage display apparatuses 39 and 40 in front of eyeballs 34R and 34L ofan observer, respectively. Since an interpupillary distance 38 of theobserver varies depending on their individuals, an amount ofinterpupillary distance adjustment is set to 5 mm which is a half of 10mm of an interpupillary distance. The ratio of the lateral or horizontaldimension and the longitudinal or vertical dimension of the observedimage, that is, the ratio of view angles (FovH:FovV) is 4:3.

For example, when the pupil diameter is 5 mm in the horizontal direction(lateral direction of the image), the pupil diameter in the verticaldirection (longitudinal direction of the image) can be represented as5×(FovV/FovH) mm=5×(3/4). When the pupil diameter of the observer in thehorizontal direction is 3 to 5 mm and the amount of the interpupillarydistance adjustment is a half of 10 mm (5 mm on each side) of theinterpupillary distance, the necessary diameter of the exit pupil in thehorizontal direction is calculated as “5+5 mm=10 mm.”

The following is calculated from 10/{5×(3/4)}=2.66:

1<φH/φV<2.66   (2)

The expression (1) is rewritten with the horizontal view angle (displayview angle in the first direction) FovH and the vertical view angle(display view angle in the second direction) FovV of the observed image:

FovH/FovV<φH/φV   (1)′

The expression (2) is rewritten assuming that the pupil diameter in thehorizontal direction (lateral dimension of the image) is φ mm (φ=1 to 7mm and typically φ=3 to 5 mm):

FovH/FovV<φH/φV<(φ+5)/(φ×FovV/FovH)   (2)′

The ratio of expansions of the diffracted light in the directions 16 xand 16 y in FIG. 6, that is, the ratio of the light flux diameters isset to satisfy the expression (2) or (2)′, thereby enabling formation ofthe exit pupil having an appropriate shape and size without blocking ofthe observed image and to reduce a loss of light amount.

Embodiment 2

FIG. 10 shows a diffractive optical element 5′ for use in a retinalscanning type image display apparatus which is Embodiment 2 of thepresent invention. The diffractive optical element 5′ has a shapedifferent from that of the diffractive optical element 5 described inEmbodiment 1. The overall configuration of the retinal scanning typeimage display apparatus of Embodiment 2 is identical to that ofEmbodiment 1.

The diffractive optical element 5′ shown in FIG. 10 is of a phase type.Black portions and white portions in FIG. 10 have different heights forforming the periodic structure of the diffractive optical element.Various phase differences of the light flux are produced depending onentering positions of the light flux into each periodic portion 5 a′ ofthe periodic structure. A pitch 15 a of the periodic structure in ahorizontal direction is equal to a pitch 15 b thereof in a verticaldirection, so that diffracted light is produced at regular intervals.

FIG. 11 shows the light intensity of a diffraction pattern from thediffractive optical element 5′. As seen from FIG. 11, an area withdiffracted light of high intensity, that is, an area with high lightintensity (area in which dark black dots are arranged) has differentdimensions for a horizontal direction 17 x and a vertical direction 17y.

In Embodiment 2, the ratio of diameters of the diffracted light flux inthe horizontal direction 17 x and the vertical direction 17 y is set tosatisfy the expressions (2) and (2)′ described in Embodiment 1. FIG. 11shows the case where nine split beams with high intensity are present inthe horizontal direction and four split beams with high intensity arepresent in the vertical direction. Since φH:φV is 9:4, φH/φV is equal to2.25.

Embodiment 3

FIG. 12 shows the configuration of a retinal scanning type image displayapparatus which is Embodiment 3 of the present invention. A light source21 can emit light with modulated intensity. A light flux emitted fromthe light source 21 passes through a first optical system 22, and thenenters a two-dimensional scanning unit 23. The two-dimensional scanningunit 23 is identical to that described in Embodiment 1. The light fluxdeflected by the two-dimensional scanning unit 23 enters a secondoptical system 24.

The second optical system 24 in Embodiment 3 is formed of a mirroroptical system 24 a including two mirrors having an optical power(reciprocal of the focal length) and an optical system 24 b having atransmission/reflection function and an optical power. On the opticalpath of the deflected light flux from the two-dimensional scanning unit23 to a diffractive optical element 25, the optical system 24 b servesas a reflecting optical system.

The second optical system 24 converges the light flux from the lightsource 21 at the position where the diffractive optical element 25 isplaced, and forms a scanned surface. Light fluxes 28 a, 28 b, and 28 care examples of the light flux deflected by the two-dimensional scanningunit 23.

Preferably, the position of the diffractive optical element 25 coincideswith the position of the scanned surface. However, they do notnecessarily need to coincide with each other. That is, they only need tobe placed within an area in which they can be considered to opticallycoincide with each other.

The diffractive optical element 25 of Embodiment 3 is of a reflectiontype and has the function of diffracting the incident light flux whenthe element 25 reflects the light.

The light flux reflected and diffracted by the diffractive opticalelement 25 passes through the optical system 24 b and then enters anocular optical system 26. An observer puts his/her eye in the area wherean exit pupil 27 is formed by the light flux emerging from the ocularoptical system 26, so that he/she can observe an image formed on theabovementioned scanned surface as a virtual image through the ocularoptical system 26.

A light source control circuit 10, a scanning control circuit 11, a maincontrol circuit 12, and an image information input circuit 13 have thesame functions as those in Embodiment 1. The circuits 10 to 13 operateto provide an image in the same manner as described in Embodiment 1. Asa result, the image corresponding to the image information input from animage supply apparatus, not shown, can be presented to the observer.

FIGS. 13A to 13C show examples of the shape of the diffractive opticalelement 25 of Embodiment 3. The diffractive optical element 25 is areflective micro-lens array including a plurality of concavemicro-lenses 25 a arranged regularly as a periodic structure. Themicro-lenses 25 a are arranged with a first periodic pitch 48 a in ahorizontal direction 47 and with a second periodic pitch 48 b largerthan the first periodic pitch 48 a in an inclined direction 44 withrespect to the horizontal direction 47 and a vertical direction. Themicro-lenses 25 a are also arranged with a third periodic pitch 48 b (acombined periodic pitch of the first periodic pitch 48 a and the secondperiodic pitch 48 b) 48 c larger than the second periodic pitch in thevertical direction.

FIGS. 13B and 13C show the sectional shapes in the directions 47 and 44for the periodic pitches 48 a and 48 b, respectively. The micro-lensarray produces the diffractive effect of light with phase distributionprovided for the light reflected by the concave surface of each of themicro-lenses 25 a. A reflecting film is formed on each of lens surfaces45 a and 46 a of the micro-lenses 25 a in the sectional view of FIGS.13B and 13C.

FIG. 14 shows a pattern 49 of the diffracted light reflected by thediffractive optical element 25 shown in FIG. 13A. In FIG. 14, adirection 50 x corresponds to the horizontal direction, while adirection 50 y orthogonal to the direction 50 x corresponds to thevertical direction (the direction of combined periodic pitch directionof the first periodic pitch 48 a and the second periodic pitch 48 b).

As seen from FIG. 14, the diffracted light expands differently for thedirection 50 x and the direction 50 y such that the expansion of thediffracted light in the direction 50 x is larger than that of thediffracted light in the direction 50 y.

The different periodic pitches in the different directions in theperiodic structure on the diffractive optical element 25 can providedifferent distributions of light amount in the direction 50 x and thedirection 50 y such that the distribution in the direction 50 x islarger than the distribution in the direction 50 y. As a result, the useefficiency of light can be increased.

While the micro-lens array of the reflective type is described as thediffractive optical element 25 in Embodiment 3, it is possible to use areflective diffractive optical element configured like a phase platedescribed in Embodiment 2.

Since the diffractive effect of light is provided by the periodicstructure in the diffractive optical element 25 also in Embodiment 3,the beam diameter (spot size) D in a certain direction on the periodicstructure is desirably larger than the pitch p of the periodic structurein that direction. If the pitch p of the periodic structure isexcessively smaller than the beam diameter D, the diffraction angle perorder of diffraction is excessively increased, so that a loss of lightamount tends to occur. Thus, the following is desirably satisfied:

1<D/p<5

The beam diameter (spot size) D is defined by Full Width Half Maximum(FWHM) or 1/e² of the peak light amount.

As described above, according to Embodiments 1 to 3, the diffractiveoptical element not only increases the divergent angle in the first andsecond directions but also provides the divergent angle in the firstdirection larger than that in the second direction. Thus, especially,the size of the exit pupil can be increased in a desired direction(second direction). As a result, the retinal scanning type image displayapparatus and the head-mounted image display apparatus can be realizedwhich have the exit pupil with a shape facilitating observation of animage by an observer and which achieve the enhanced use efficiency oflight from the light source, that is, a smaller loss of light from thelight source, to enable observation of a bright image.

While each of Embodiments 1 to 3 has shown only one light source, acolor image can be presented by using a light source which emits lightin colors of red, blue, and green.

Each of Embodiments 1 to 3 has been described mainly for the purpose ofincreasing the use efficiency of light from the light source by settingthe shape of the exit pupil to have different dimensions for the twodirections orthogonal to each other. It is also possible to preventoccurrence of stray light by precluding light from proceeding indirections other than the necessary direction.

Furthermore, the present invention is not limited to these embodimentsand various variations and modifications may be made without departingfrom the scope of the present invention.

This application claims foreign priority benefits based on JapanesePatent Application No. 2006-219685, filed on Aug. 11, 2006, which ishereby incorporated by reference herein in its entirety as if fully setforth herein.

1. A scanning type image display apparatus comprising: a scanning unitwhich two-dimensionally scans a light flux; a first optical system whichintroduces a light flux from a light source to the scanning unit; asecond optical system which converges the light flux from the scanningunit; a diffractive optical element which receives the converged lightflux from the second optical system; and an ocular optical system whichintroduces the light flux from the diffractive optical element to an eyeof an observer, wherein, when two directions orthogonal to each other indiametral directions of the light flux from the second optical systemare a first direction and a second direction, the diffractive opticalelement has a function of increasing divergent angles o an emerginglight flux from the diffractive optical element in the first and seconddirections as compared with convergent angles of an incident light fluxentering the diffractive optical element in the first and seconddirections and a function of increasing the divergent angle of theemerging light flux in the first direction as compared with thedivergent angle thereof in the second direction, wherein the firstdirection corresponds to a direction of a long side of a display imageand the second direction corresponds to a direction of a short side ofthe display image.
 2. The apparatus according to claim 1, wherein ascanned surface of the light flux scanned by the scanning unit is formedbetween the second optical system and the ocular optical system, andwherein the diffractive optical element is placed in an area where thescanned surface is formed.
 3. The apparatus according to claim 1,wherein the following condition is satisfied:1<φH/φV<2.66 where φH represents a width of the emerging light flux fromthe diffractive optical element in the first direction, and fVrepresents a width of the emerging light flux in the second direction.4. (canceled)
 5. The apparatus according to claim 1, wherein thediffractive optical element has a periodic structure in the first andsecond directions, and wherein a periodic pitch of the periodicstructure in the first direction is smaller than that in the seconddirection.
 6. The apparatus according to claim 1, wherein thediffractive optical element has a periodic structure in the first andsecond directions, and the diffractive optical element producesdifferent phase differences of the light flux depending on enteringpositions thereof into each periodic portion of the periodic structure.7. An image display system comprising: the scanning type image displayapparatus according to claim 1; and an image supply apparatus whichsupplies image information to the scanning type image display apparatus.